1. Field of the Invention
The present invention relates to a thermal recording control method and system which control the thermal recording format of a thermal recorder used as a recorder in a facsimile equipment or the like and, more particularly, to a thermal recording control method and system which is suitably employed in such a thermal recorder that a line corresponding to a picture data is repeatedly recorded in the unit of plural lines in a feed direction to obtain a desired picture.
2. Description of the Prior Art
For example, in the case of a thermal recorder used as a recorder in a facsimile equipment. Such repetitive recording of every plural lines (usually, 2 lines) as mentioned above is carried out in a specific operational mode that is generally referred to as `normal mode`.
More specifically, for example, in the event where a thermal head having a heater of a size allowing the recording of lines at a pitch of 7.7 lines/mm in the feed direction is used as a thermal member to record picture data being transmitted at a pitch of 3.85 lines/mm (which operational mode is usually referred to as `normal mode`), adjustment of recording pitch is realized by recording an identical picture data by every two lines as shown in FIG. 1.
In FIG. 1, a zone shown by slanted lines corresponds to a black picture element, that is, the heater of the thermal head is heated in the zone. Further, reference symbol .DELTA.t.sub.1 denotes a recording time of the thermal head necessary for the recording of one line, symbol .DELTA.t.sub.2 denotes a recording time of the thermal head necessary for the recording of two lines, and these recording times .DELTA.t, and .DELTA.t.sub.2 usually satisfy a relationship which follows. EQU .DELTA.t.sub.1 =.DELTA.t.sub.2 /2 (1)
Meanwhile, the temperature rise and drop rates of a heater in a thermal head are generally determined by the type, model and so on of the thermal head. And when a single heater is continuously driven and heated, the temperature of the heater varies with time, e.g., following such a curve as shown in FIG. 2(a). Therefore, assuming, for example, that the aforementioned thermal head can record one line in 5 msec (.DELTA.t.sub.1 =5 msec), then picture data being transmitted at a pitch of 7.7 lines/mm can be recorded at a rate of one line/5 msec (which operational mode is usually referred to as `fine mode` by contrast with the `normal mode`), whereas picture data being transmitted at a pitch of 3.85 lines/mm as in the above case are recorded at a rate of one picture data line/10 msec (.DELTA.t.sub.2 =10 msec) because the data must be recorded in multiples of 2 lines.
That is, with respect to the picture data being transmitted at a rough rate of 3.85 lines/mm that is twice rougher than 7.7 lines/mm, it is also desirable to realize the reduction of a relative recording time of one line of the picture data to 5 msec (the actual recording rate being 2 lines/5 msec). However, this realization is difficult in actual applications for reasons which follow.
In general, a recording energy E [mJ/dot] of the aforementioned heater is expressed as follows. EQU E=V.sup.2 .multidot..DELTA..tau./R (2)
where V denotes a voltage applied to the heater (head application voltage), R denotes the resistance of the heater, and .DELTA..tau. denotes an active (drive) time of the heater (the pulse width of a recording pulse) respectively. Among these factors, .DELTA..tau. is usually used as a parameter of controlling the recording energy E.
Conventionally, it has been common practice to control the pulse width .DELTA..tau. of the recording pulse according to the temperature rise of the entire thermal head (in particular, the heater substrate) detected by a temperature sensor disposed within the thermal head. FIG. 3 shows an example of how to set the pulse width of the recording pulse with respect to the temperature of such a thermal head.
According to such a method of controlling the recording energy E, however, in the case where, as mentioned above, picture data being transmitted at a pitch of 3.85 lines/mm are recorded in the unit of two lines with use of a thermal head allowing the recording of data at a pitch of 7.7 lines/mm an at a rate of one line/5 msec, if the recording time of one picture data line is set to be 5 msec, i.e., .DELTA.t.sub.2 is set to be 5 msec, then the recording time .DELTA.t.sub.1 corresponding to one of actually recorded lines must be set to be 2.5 msec. In this case, it becomes impossible to provide a sufficient cooling time to the heater and heat accumulation quickly takes place in the heater as shown in FIG. 2(b) in contrast with FIG. 2(a), which leads to so-called a `trailing` phenomenon, with the result that the recorded picture quality is remarkably deteriorated. And such a phenomenon cannot utterly be avoided by such a recording energy control method based on the detection of the heater substrate temperature as mentioned above.